At this point, I often find that people are prone to proposing that some unknown mechanism exists that scientists haven’t found yet. This is, however, a logical fallacy known as ad hoc. You can’t just make up an unknown mechanism whenever it suits you. If that was valid, then you could always reject any scientific result that you wanted, because it is always possible to propose some unknown mechanism. Similarly, you can’t use the fact that scientists have been wrong before as evidence, nor can you argue that, “there are still things that we don’t understand about the climate, so I don’t have to accept anthropogenic climate change” (that’s an argument from ignorance fallacy). Yes, there are things that we don’t understand, but we understand enough to be very confident that we are causing climate change, and, once again, you can’t just assume that all of our current research is wrong.
The key problem here is the burden of proof. By claiming that there is some other natural mechanism out there, you have just placed the burden of proof squarely on your shoulders. In other words, you must provide actual evidence of such a mechanism. If you cannot do that, then your argument is logically invalid and must be rejected.
Summary/Conclusion
Let’s review, shall we?
We know that it’s not the sun
We know that it’s not Milankovitch cycles
We know that it’s not volcanoes
We know that even when combined, natural causes cannot explain the current warming
We know that CO2 traps heat
We know that increasing CO2 causes more heat to be trapped
We know that CO2 was largely responsible for past climate changes
We know that we have roughly doubled the CO2 in the atmosphere
We know that the earth is trapping more heat now than it used to
We know that including anthropogenic greenhouse gasses in the models is the only way to explain the current warming trend